Epithelial Mesenchymal Transition
TGF-β & Cell-Cell Contact Model
The epithelial mesenchymal transition (EMT) is a process by which carcinoma cells lose adhesion and gain the invasive and migratory properties of mesenchymal cells. This transition allows the transformed cells to migrate away from a primary tumor while maintaining their newly acquired invasive behavior, suggesting that there is a bistable switch between the epithelial and mesenchymal phenotypes. In recent experimental work, we have shown evidence of this bistability in the MCF7 breast carcinoma cell line. Underlying the complex processes governing EMT, we identify a double-negative feedback loop between E-cadherin, a protein involved in cellular adhesion, and Slug, a transcription factor that is upregulated by EMT and suppresses E-cadherin production. In the current work, we present a simple mathematical model that examines the relationship between E-cadherin and Slug in response to pro-epithelial and pro-mesenchymal factors, cell-cell contact and exogenous TGF-β, respectively. We hypothesize that the transition between the epithelial and the mesenchymal steady states in MCF7 cells is a set of coupled bistable switches: the bistable switch underlying a loss of cell-cell contact is reversible, but the bistable switch in response to exogenous TGF-β is not. An epithelial cell that transitions into a mesenchymal cell when exposed to sufficient TGF-β will be able to maintain this phenotype even after it loses exogenous signal, allowing it to migrate away from primary tumor. Further, with these coupled bistable switches, this model shows that it is possible for an MCF7 cell with an initially high level of cell-cell contact to transition into a mesenchymal cell if the exogenous TGF-β level is increased while cell-cell contact is simultaneously lost. The predictions of this model for E-cadherin and Slug levels were compared against relative gene expression data from our recent experiments with MCF7 cells. Our model works well to predict E-cadherin and Slug mRNA expression in low confluence experiments, while also highlighting some aberrations between experimental data and theoretical predictions.
Work performed in collaboration with Dr. Marlene Hauck and Dr. Sudin Bhattacharya.
MCF7 without TGF-b
MCF7 with TGF-b
SW480 with TGF-b
MCF7 without TGF-b
This experimental work examined the dual influence of the TGF-β pathway and intercellular contact on the activation of EMT in colon (SW480) and breast (MCF7) carcinoma cells by using immunocytochemistry staining, flow cytometry, qPCR, and protein extractions. While the SW480 population revealed an intermediate state between the epithelial and mesenchymal states, the MC7 cells exhibited highly adhesive behavior. However, for both cell lines, an exogenous TGF-β signal and a reduction in cellular contact pushed a subgroup of the population towards the mesenchymal phenotype. Further, the subgroup of cells most likely to exhibit mesenchymal like properties when exposed to exogenous TGF-β were those with few cellular contacts. Together, these results highlighted that, while EMT is induced by the synergy of multiple signals, this activation varies across cell types.
Work performed in collaboration with Nikki Wagner, Dr. Jhon Cores, Rose Caspar,
Dr. Alyson Wilson, Dr. Sudin Bhattacharya, and Dr. Marlene Hauck.
Wnt Signaling Model
Following the formation of a primary carcinoma, neoplastic cells metastasize by undergoing the epithelial mesenchymal transition (EMT), which is triggered by cues from inflammatory and stromal cells in the microenvironment. We hypothesize that a bistable switch between the epithelial and mesenchymal phenotypes governs EMT, allowing the cell to maintain its mesenchymal phenotype even after it leaves the primary tumor microenvironment and EMT-inducing extracellular signal. This work presented a simple mathematical model of EMT using ordinary differential equations (ODEs), specifically the roles played by four key proteins in the Wnt signaling pathway: Dishevelled (Dvl), E-cadherin, β-catenin, and Slug. The model predicts that following activation of the Wnt pathway, an epithelial cell in the primary carcinoma must attain a threshold level of membrane-bound Dvl to convert to the mesenchymal-like phenotype and maintain that phenotype once it has migrated away from the primary tumor. Furthermore, sensitivity analysis (Latin Hypercube Sampling, Partial Rank Correlation Coefficient) of the model suggests that in both the epithelial and the mesenchymal states, the steady state behavior of E-cadherin and the transcription factor Slug are sensitive to changes in the degradation rate of Slug, while E-cadherin is also sensitive to the IC50 (half-maximal) concentration of Slug necessary to inhibit E-cadherin production. The steady state behavior of Slug exhibits sensitivity to changes in the rate at which it is induced by β-catenin upon activation of the Wnt pathway. In the presence of sufficient amount of Wnt ligand, E-cadherin levels are sensitive to the ratio of the rate of Slug activation via β-catenin to the IC50 concentration of Slug necessary to inhibit E-cadherin production. The sensitivity of E-cadherin to the degradation rate of Slug, as well as the IC50 concentration of Slug necessary to inhibit E-cadherin production, shows how the adhesive nature of the cell depends on finely-tuned regulation of Slug. By highlighting the role of β-catenin in the activation of EMT and the relationship between E-cadherin and Slug, this model identifies critical parameters of therapeutic concern, such as the threshold level of Dvl necessary to inactivate the GSK-3β complex mediating β-catenin degradation, the rate at which β-catenin translocates to the nucleus, and the IC50 concentration of Slug needed to inhibit E-cadherin production
Work performed in collaboration with Dr. Alyson Wilson, Dr. Marlene Hauck, and
Dr. Sudin Bhattacharya.